The effects of dilute additions of deuterium (D) and tritium (T) on the elastic moduli of single crystals of Nb and V were measured at room temperature. The coefficients &Dgr;C44/C44and &Dgr;C′/C′, per at.% isotope, are evaluated and compared with published data for H additions and previous measurements of D additions. The agreement among data from different laboratories is well within experimental error. The &Dgr;C′ effects in Nb are isotope dependent and closely related to the square root of the inverse mass. In the case of V, the &Dgr;C′ effect is the same for T and H but smaller for D, suggesting a possible change from tetrahedral to octahedral site occupancy with increasing isotope mass.